The mechanism of formation of the high molecular weight peptides, liberated by proteolytic enzymes from the C-terminal half of the alpha-chain of fibrinogen, was investigated. Simultaneous cleavage at a number of susceptible sites results in heavier intermediates and a lighter, relatively stable endproduct. The mechanism appears to be similar with plasmin and trypsin, however, the intermediates are shorter-lived and the relatively stable endproducts of lower molecular weight with trypsin than with plasmin.